<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        div{
            width: 100px;
            height: 100px;
            background-color: green;
        }

/*  假设页面宽度=500 */
        /* 写在style 标签里面 */
        /* max-width  小于等于800  （从大到小）*/
         /* min-width  大于等于800  （从小到大）*/
        /* width  等于 */
        @media screen and (max-width: 600px){
            div{
                background-color: blue;
            }
            html{
                font-size: 25px;
            }
        }
        @media screen and (max-width: 800px){
            div{
                background-color: red;
            }
        }

       

    </style>
</head>
<body>
    <div>

    </div>
</body>
</html>